import java.util.Arrays;

public class MyStack {
int[] array;
int size;
public MyStack(){
array = new int[3];
}
public int push(int e){
ensureCapacity();
array[size++] = e;
return e;
}
public int pop(){
int e = peek();
size--;
return e;
}
public int peek(){
if(empty()){
throw new RuntimeException("栈为空，无法获取栈顶元素");
}
return array[size-1];
}
public int size(){
    return size;
}
public boolean empty(){
        return 0 == size;
}
private void ensureCapacity(){
        if(size == array.length){
            array = Arrays.copyOf(array, size*2);
        }
    }
}
